1. Field of the Invention
This invention generally relates to the method and apparatus for manufacturing substrates having fine circuit patterns, such as semiconductor devices or liquid crystals, especially to the technology for the inspection of patterns for semiconductor devices or photo masks. Further in particular, it relates to the inspection method and apparatus using a charged particle beam for the inspection of defects in any portion on an unfinished semiconductor wafer in the manufacturing process of a semiconductor device.
2. Description of the Related Art
A semiconductor device is produced by repeating the processes of printing a pattern formed on a photo-mask onto a wafer through lithography and etching processes. For the inspection of such patterns, there is a technology of identifying defects in a wafer pattern by acquiring a SEM image (an image of potential contrast) of the wafer pattern and then comparing it with an image of neighboring another pattern. Since the picture quality of a voltage contrast image is seriously affected by charge conditions of the object of inspection, charge control is very important with the wafer-pattern inspection technology using a SEM image.
For charging a wafer, 1) the method of irradiating an electron beam to a wafer, and 2) the method of using a charge control electrode, are the measures to be used. The principles of these methods are explained below.
In the former method 1), the electron energy of an electron beam is controlled so that the efficiency of secondary electron emission generated by the irradiation of an electron beam to a wafer reaches a predetermined value. The charge polarity is determined by the efficiency of the secondary electron emission; if the efficiency of the secondary electron emission is larger than 1, the charge will be positive, and if smaller than 1, the charge will be negative. The relation between the efficiency of the secondary electron emission and electron energy is shown in FIG. 7.
In the latter method 2), the charge polarity is controlled by controlling the potential applied to the charge control electrode relative to the wafer. If the charge control electrode is charged with a positive voltage relative to the wafer, the wafer will be positively charged, and if with a negative voltage, the wafer will be negatively charged. That is, if an electrical field that will draw up the secondary electrons emitted from the wafer field is generated, the wafer will be charged positive, and if an electrical field that will draw back the electrons is generated, the wafer will be charged negative.
When charging the surface of a wafer, it is necessary to charge an inspection area evenly. If the wafer surface is unevenly charged, the trajectory of the secondary electron is largely bent, and the detection efficiency of secondary electrons is affected. As the result, for example, obtained images could be distorted and have fluctuated or uneven brightness and the defect inspection accuracy will be deteriorated. As for the causes of uneven charge in the wafer surface,    (1) The case that the method of generating negative charge is the cause,    (2) The case that the circuit pattern formed on the wafer surface is the cause,are the possible cases.
These two cases are explained below;
First, the reason the method of generating negative charge can be the cause of an uneven charge is explained. A method of using a charge control electrode for generating a negative charge is described in the JP H11-121561A.
As shown in FIG. 5, in the method described, by charging a negative voltage to an electrode (charge control electrode 807) set up just above a wafer, a potential barrier is formed just above the wafer so that the secondary electrons 809 generated by irradiating an electron beam 801 are pulled back to the wafer and adhere to the surface of the wafer, which thereby gets charged negatively. In this method, only the area irradiated by the electron beam gets negatively charged, so that an electrical field is generated between the irradiation area and its surrounding area that is not negatively charged. This electrical field spreads the electrons adhering to the wafer surface, eliminating the difference in potentials and thus difference in contrast between a defective portion and a normal portion, and degrading inspection accuracy.
Next, with reference to FIG. 5 and FIG. 6, the reason why a circuit pattern formed on a wafer surface causes uneven charging will be explained. As shown in FIG. 6, suppose an inspection area on the left half of which plugs are formed as a circuit pattern and the right half of which has no pattern.
When the secondary electrons are pulled back onto a pattern like this, plugs 804 in FIG. 5 do not charge because they are conductive to the substrate 802, but the insulating film 803 charges negatively. With the pattern shown in FIG. 6, the left half having dense plugs has a higher charge potential than the right half having no pattern. Because of the difference in charge potentials between the left half and the right half, an electric field is generated and the secondary electron trajectories are bent. Therefore, when a voltage contrast image 902 is acquired for the wafer pattern 901, shading occurs, which degrades the defect inspection accuracy.
With the cases (1) and (2) explained above, uneven charges occurred at the time of performing a pattern inspection. However, a wafer can already have a charge before being inspected with an inspection apparatus. Such a charge, as being the result of the wafer's coming through various fabrication processes, is uneven and hard to tell even whether positive or negative.
As a method of charge control to even an uneven charge, a pre-charge method using ultraviolet or electron beam irradiation is known.
A charge control method using ultraviolet irradiation is, for instance, described in JP 2003-151483A. In this technology, an even charge is attained by removing electrons accumulated in a sample through ultraviolet irradiation that causes photoelectric emission or renders the surface of the sample conductive.
Another method of charge control by pre-charging is disclosed, for example, in JP2000-208085A. In the invention described in JP2000-208085A, the surrounding area of an image acquiring target area is negatively charged (pre-charge) before acquiring an image using an electron source different from the electron source for acquiring images. Then, by irradiating an electron beam emitted from the electron source for image acquisition, an image is acquired. The electron energy of the electron beam emitted from the electron source for pre-charging is controlled so that the emission efficiency of the secondary electrons is below 1.